1. Field of the Invention
The present invention relates to a golf club shaft, more particularly to a golf club shaft having a feeling similar to the feeling of a steel shaft and being superior in stability.
2. Prior Art
FIG. 8 is a perspective view showing a configuration of a conventional plastic golf club shaft. As shown in FIG. 8, the golf club shaft has a structure having a torsional rigidity holding layer 1 in which reinforcing fibers are diagonally crossed, a flexural rigidity holding layer 2 in which reinforcing fibers are aligned in a direction parallel with the longitudinal direction of the shaft, and optionally a compressive rigidity holding layer 3 in which reinforcing fibers are aligned in the direction vertical to the longitudinal direction of the shaft. Typically, the golf shaft is formed by 4 to 6 plies of the torsional rigidity holding layer 1 and 4 to 6 plies of the flexural rigidity holding layer 2 (e.g. Specification of Japanese Patent Application No. 311678/1995).
In the case of a conventional plastic shaft, optionally a prepreg in which reinforcing fibers are aligned in the direction vertical to the longitudinal direction of the shaft is wound on a tapered shaft-like metallic mandrel. Thereafter, a prepreg sheet 4 in which reinforcing fibers are diagonally crossed is wound on the above mentioned prepreg layer. As shown in FIG. 9, the prepreg sheet 4 is made such that overlapping a titled prepreg 41 in which reinforcing fibers such as carbon fibers are diagonally set in a predetermined direction with an incline prepreg 42 in which reinforcing fibers are set in the direction opposite to the predetermined direction. Then a prepreg sheet in which reinforcing fibers are set in the direction parallel with the longitudinal direction is wound on the prepreg sheet 4, then a tape is spirally wound on the prepregs for setting, and a thermosetting resin contained in the prepreg sheets is thermally cured. Hereafter, a prepreg in which reinforcing fibers are aligned in the uni-direction is referred to as a UD prepreg. In this case, the concept of the UD prepreg includes not only the prepreg in which reinforcing fibers are aligned in a direction parallel with and vertical to the longitudinal direction of the shaft but also the incline prepreg 41 in which reinforcing fibers are set on a slant to a predetermined direction and the titled prepreg 42 in which reinforcing fibers are set the direction opposite to the predetermined direction.
In the case of the golf club shaft manufactured in accordance with the above method, a tape trace for setting is formed on the surface of the shaft. Therefore, the shaft is formed into a product by polishing the surface of the above outermost-surface flexural rigidity holding layer, removing the tape trace and smoothing the surface, applying painting and printing to the surface, and then forming a transparent surface layer.
The above plastic shaft is basically manufactured by curing the thermosetting resin contained in the UD prepreg layer in which reinforcing fibers are aligned in one direction as described above. However, though a reinforcing fiber (in the case of carbon fiber) has an elongation of 1.5%, a plurality of thermosetting resin layers has a small strength and a large flexibility compared to the reinforcing fiber. Therefore, the thermosetting resin layer shows a sufficient effect in the direction in which reinforcing fibers are aligned. However, it has a problem that a deformation or displacement occurs between thermosetting resin layers when a force is applied in the thickness direction or transverse direction. When taking a shot by a club using the golf shaft manufactured as described above, a problem occurs that a stable shot cannot be easily taken due to a displacement or deformation between thermosetting fiber layers. Therefore, a fluctuation may occur in direction and carry. Moreover, the above displacement between thermosetting resin layers may deteriorate the feeling of a shot. That is, though a golf senior tends to like the feeling of a steel shaft, the above displacement between thermosetting resin layers has a problem that it causes a feeling separate from the feeling of a steel shaft.
Moreover, the torsional rigidity holding layer 1 is formed by adhering the UD prepregs 41 and 42. So it has a problem that accuracy of shaft is not improved due to a displacement for laminating the prepregs. Furthermore, because laminating is performed, a problem occurs that the number of steps increases and the workability is deteriorated. Hereafter, the above torsional rigidity holding layer is referred to as a UD torsional rigidity holding layer.